BLUFF EROSION ON OUTER CAPE COD: SHORT-TERM EROSION RATES AND OBSERVATIONS OF CONTROLLING FACTORS

Bluff top retreat was measured quarterly for two years at four sites on Outer Cape Cod beginning in December of 2000. These sites have historical significance and include Cape Cod Light, Marconi Station, Nauset Light and Coast Guard Beach. Short-term bluff top retreat rates varied from 0.95 m/yr to 3.5 m/yr. Despite relative mild weather conditions, these rates are high compared with other short-term as well as long-term studies, e.g., a long-term rate of .8 m/yr derived from existing data for the entire Outer Cape (Giese and Aubrey, 1987). Maximum quarterly retreat for a single transect was 4.5 meters.

Observations of factors that affect bluff erosion rates were made at the time of each measurement. These included beach width, beach volume above MLLW, average beach elevation and groundwater seepage from the bluff face. Bluff height (as a control on the amount of available talus) and bluff stratigraphy observations were made as well and supplemented with existing stratigraphic data. In addition, wave energy and precipitation data from the study period were compared to the erosion data for each site.

While the smooth curvature of the Outer Cape shoreline suggests that, in the long run, sea level rise and large storms control the long-term erosion pattern, results from this study suggest that terrestrial processes may dominate locally over the short-term. The relative influence of terrestrial processes appears to vary spatially and temporally and control much of the local geomorphic irregularities. Such short-term processes include incising due to surface runoff, slumping due to groundwater seepage and erosion due to aeolian transport. Bluff stratigraphy appears to play a significant role in controlling the variability in terrestrial short-term erosion from one site to another.